"We think it’s a good idea to encourage people, but not so fast," he writes.

Surprisingly, both pessimists and optimists perform at the same rate, but their strategies for attaining success are different. Optimistic people set high expectations and benefit from confidence, whereas pessimistic people set lower expectations but their anxiety and negative thinking push them to try harder, according to a series of studies published in the Journal of Personality and Social Psychology.

The study says that "positive mood impairs the performance of defensive pessimists."

"The encouragement boosted their confidence, quelling their anxiety and interfering with their efforts to set low expectations," Grant says. "When they’re in a good mood, they become complacent; they no longer have the anxiety that typically mobilizes their effort. If you want to sabotage defensive pessimists, just make them happy."

Want your business advice featured in Instant MBA? Submit your tips to tipoftheday@businessinsider.com. Be sure to include your name, your job title, and a photo of yourself in your email.